Rotary pumps having pumping elements consisting of a driving inner rotor and a driven outer rotor are generally referred to as internal gear pumps. A particular class of internal rotary gear pumps commonly known as internal gerotor pumps are often used in chemical metering applications, for example, when pumping the components of two-part polyurethane foam.
Gerotor type pumping elements are characterized by an inner member having one less tooth than the outer member and by the fact that each tooth of one member is always in contact with some portion of the other member. This interaction between the members results in continuous driving contact, and when the gears are rotated, a series of expanding and contracting chambers are formed which, when connected with appropriate passages, provides pumping action. In the case of the conventional externally generated gerotor, the outer member has a series of inwardly protruding circular teeth such that the set has the aforementioned properties.
The limitations of this approach are apparent in the foam-in-place packaging industry where two-part polyurethane is used to make the packaging materials. The two-part polyurethane foam packaging material is based upon the reaction of two precurser components, which when mixed will react to form a polymer foam and gaseous by-products. In particular, and most commonly, an isocyanate containing component is mixed with a polyol containing component and these components react to produce a urethane polymer (polyurethane), steam, and carbon dioxide.
As the two-part polyurethane foam requirements have become more specialized in the foam-in-place industry, the constituent parts, i.e., the isocyanate containing component and the polyol containing component, have become more abrasive and less viscous. Because of the inherent sliding action in a conventional externally generated gerotor set, pump life in this particular application has been reduced from over 1000 hours to about 100-200 hours.
A lesser known form of the conventional gerotor is the IGR or Internally Generated Rotor Set. In this device, the inner rotor has a number of circular externally protruding teeth and the outer rotor is internally generated such that it has the same characteristics as an externally generated rotor set, i.e., an inner member having one less tooth than the outer and where each tooth of one member is always in contact with some portion of the other member, thus resulting in continuous driving contact. In this case, however, the circular teeth of the inner member can be replaced by rolls which are contained in recesses in the inner member, which recesses are of substantially the same diameter as the rolls. Properly designed, this allows the rolls to operate hydrodynamically within the recesses. Further, the combination of both centrifugal and pressure forces drive the roll into intimate rolling contact with the outer member, thus providing fluid tight sealing as well as the elimination of the sliding contact that has led to the reduction in pump life in conventional gerotor pumps as the pumped chemicals have become more abrasive and less viscous. It should be noted that because of the pressure loading of the rolls, the IGR will accommodate a certain amount of tooth wear without a loss in pumping performance. An example of an internally generated rotor set is disclosed in U.S. Pat. No. 3,623,829.
Rotary pumps using internally generated rotor sets of the foregoing types are characterized by the fact that the lobe outline of the inner rotor is centered on an axis spaced from and parallel to the axis on which the recess outline of the outer rotor is centered, this spacing being termed the “eccentricity.” One cycle is defined as the rotation required for the inner rotor to advance one lobe in relation to the outer rotor, and the total volumetric expansion (or contraction) of the spaces between gear lobes of a specified thickness in one cycle is termed the “displacement” of the rotor set. Internally generated rotor sets are not known to have been previously used in chemical metering applications.
In a practical device using an internally generated rotor set of the foregoing type, there are a number of ways of supporting the rotors. Both rotors may be rotated about fixed axes, or either of the rotors may be held fixed while the other rotor is rotated and orbited in relation to it. As between these alternatives the choice is determined to some extent by end use considerations. In a chemical metering application it is generally desired that rotors rotate about fixed axes.